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ADS7865: Recommended the value of capacitance at input pin and protection for common mode noise

Hirotaka Matsumoto
Guru 27525 points
Part Number: ADS7865

Hi all

Would you mind if we ask ADS7865?

<Question1>
How much is the recommended the value of capacitance at input pin?
Is our following recognition correct?
The capacitor in the RC kickback filter is typically selected to be >10x larger that the sample-and-hold capacitor to provide instantaneous charge transfer to the sample-and hold,
and minimize the effects of charge kickback.
->2pF × 10 = more than 20pF?
The customer's condition is following
Construction : Buffer amp -> RC filter -> ADC input
Input typ : single end

As the background of this question, our customer changes from 2200pF to 220pF at input capacitance.
According to conversion timing, the data decreased, so does it depends on time constant also?

<Question2>
Our customer tested common mode noise test with ±2.0kV/10min.
As the result,it occurred sudden change(decrease the data) phenomenon.
If you have the measure for common mode noise(circuit, layout, choke coil?), could you share us?
The customer would like to know it as reference.

Kind regards,

Hirotaka Matsumoto

  • 0
    TI__Mastermind 49405 points

    Hi Matsumoto-san,

    The RC kickback filter selection for the SAR ADC is dependent on a number of factors, including the value of the SAR ADC internal sample-and-hold capacitance, the acquisition time, and the driver amplifier used. In most cases, the complete amplifier, RC kickback filter and SAR ADC circuit is optimized using simulation.  The complete driver amplifier circuit and RC kickback filter needs to be stable and settle within the allowed acquisition time.  The 10x or 20x suggested capacitor value for the kickback filter is a rule of thumb used only as a starting point in the design process, and this capacitor value is in some cases adjusted during the circuit optimization (the optimization may be performed using simulation).

    There is a series of videos covering this topic. TI Precision labs that covers driver amplifier selection, as well as the RC kickback filter selection with the use of TINA Spice Simulations:

    https://training.ti.com/ti-precision-labs-adcs

    If you can please provide an schematic, and/or and let us know the driver amplifier that the customer is using, the maximum sampling rate required in the application and the exact ADS7865 configuration required we could offer suggestions for the driver circuit.

    Regarding question (2), I have not performed common-mode tests on the ADS7865; if you have the specific test conditions the customer is looking for;  I could see if a relevant reference design examples is available.

    Thank you and Best Regards,

    Luis Chioye  

  • 0 in reply to Luis Chioye
    Guru 27525 points

    Luis san

    Thank you so much for your reply!
    We will send you detail information using E2E private message.
    Because the circuit data contains customer's confidential contents.
    So, could you give us the advice?

    Kind regards,

    Hirotaka Matsumoto

  • 0 in reply to Hirotaka Matsumoto
    TI__Mastermind 49405 points
    Hi Hirotaka-san,

    Thank you for the schematic. I will continue this query via E2E private message.

    Thank you,
    Kind Regards,

    Luis Chioye